Method of milling an undercut slot in a screw head



y 5. 1956 R. M. VAUGHN 2,745,120

METHOD OF MILLING AN UNDERCUT SLOT IN A SCREW HEAD Original Filed July 11, 1949 32 3/ a3 a7 :7 25 z;

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United States Patent MILLING AN UNDERCUT sLor 1N A SCREW HEAD Rudolph M. Vaughn, Newport Beach, Calif.

Original application July 11, 1949, Serial No-104,033,

now Patent No. 2,677,985, dated May 11, 1954. Divide'd and this application November 16, 1953, Serial No.395 ,199

METHOD or 3 Claims. (Cl. -10) This invention relates to a method for machining an I undercut slot with a conformation which overcomes the r the cutter of Fig. 1 is made;

, 2,745,120 Patented May 15, 1956- ICC be better understood from the following detailed description and the accompanying drawing of which:

Fig. lis a side elevation view of a screw blank held in a jig, and'having a transverse slot cut in its head by a rotary cutter; i s Fig. 2 is a perspective view of a blank from which Fig. 3 is a cross-section view taken- F ig. 2; p g I Fig. 4 is a perspective view showing part of a cutter I made from theblank of Fig. 2;

usual component of force tending to push a driver out w of. a screw slot when turning torque is applied by a.

point contacts between the driver and the Walls of the.

screw slot while driving, the two opposite forward points or edges of the driver being in contact with corresponding surfaces of the slot. This tends to wear down and round olfthe driving edges and points of the driver and results in a component of force tending to push the driver out of the slot.

Attempts have been made to overcome this usual tendency of a driver to be forced out of a slot while driving. One expedient, for example, has been to undercut the slot somewhat to overcome the effect of an outward the ordinary type of slotted screwhead,

Fig. 5 is a plan view of the slotted head of the screw of Fig. 1, illustrating the making of special additional cuts in accordance with my invention;

Fig. 6 is an elevation view, partially in cross section, taken at line 66 of Fig. 5;

Fig. 7 is-another elevation view partially incross sec tion taken at line 77 of Fig. 5, this views'howing the screw threads formed in the blank;

Fig. 8 is a front elevation view of a driver usefulfor' driving thescrew according to my invention;

Fig. 9 is an end elevation view ofthe driver. l The configuration of the slotted screw head according to this invention can be readily understood bydescribing how it can be made; although it should-be understood, that the invention is not limited to the particular method by which the slot is made. 1 T

;Referring to Fig. 1, this shows a screw blankQlQofhd ordinary well known flat head type. An a'rcuate 'slot 11 is? providedacross the head of the screw, and this can con veniently be done by an operation of a rotary milling the disadvantages of the previous types of slotted heads by the provision of a unique form of slot wherein contact between the driver and the slot.walls is.had.oyer a sjb'stantial surface area-'of' the slot walls, rather than m'erelylat points or edges. My novel slot has an arcuate base and divergesfontward from the center. Furthermore, it is undercut at the sides where the driving torque is applied, though'not at-the center.- The slot takes a driver having any arcuate bottom and sides which diverge toward the bottom. A feature is the provision'of surface contact'with the driver at opposite sides of the center of the'slot;

"This is a division of my copending application, Serial Number 104,033, filed July 11, 1949, now United States v Patent 2,677,985, dated May 11, 1954'.

' Afeature of the present invention resides in the use of a'rotary milling cutter having cutting edges on opposite sides of the cutter which diverge toward the periph- 'ery thereof. A slotis cut to a desired depth in the screw head with such a milling cutter by forcing the rotating cutter into'the screw head, and then the axisof rotation of the cutter is turned through an angle on either side of that slot was toundercut the'slot walls.

The foregoing and other features of my invention will cutter 12, the center 13 of which can be brought'straight down symmetrically along the longitudinal center axis! of the screw blank so as; to cut the slot symmetrically along a diameter of the screw head. .i'

- Cutter 12., shown partially in perspective in Fig.2, is a milling cutter which can be made from 'a blankin the general form of a circular disc (Fig. 3) on each'side' of the periphery of which there is'removedan annular portion, these being numbered 19 and 19a respectively; Eachof these portions 19 and 19a is a surface of 're'vol u'- tion, and their peripheral areas are frusto-conical, providing cross sections bounded by straight. lines ZtLand 2.0a, respectivey, extending from the respective edges of the periphery and converging inwardly toward the center. The extent of these straight portions 20 and 20a. should, be sufficient so that the radial distance d will be at least as great, and preferably greater, than the depth of theslot. Inside the straight portions 20 and 20a, the surfaces are then brought out to the flat surfacesof the discalonglines which may be curved as at 21 and 21a, respec' tively. The thickness w of the disc, at its periphery; should correspond with the width of the straight vertical sided'slot to be cut in the screw head. f

'The teeth 50 are cut in the periphery of the blank as shown in Fig. 2, so that the slot which is cut in the screw head will conform with the shape of the periphery of the blank. Accordingly, the leading cutting edges 51 and 51a of the teeth conform with the shapeof surfaces 20 and 20a of the blank. The transverse cutting edges 15 of the teeth are straight lines extending transverse to the. axis of the cutter; and since the length of the edges 15, being the same as the width w in Fig. 4, is the widest part of the teeth,'they will determine the width of the slot. The leading edges 51 and 51a act as generators of oppositely directed surfaces of revolution, in this case, conical surfaces, which are disposed at opposite edges and opposite. ends of the slot, each such surface having aicontour such as surface 46a (Fig. 9). The surfaces so oppositely disposed have a common central axis, which is the axis of rotation of the milling cutter when his turned so as to give the desired undercuttings. Accordingly, tltiere will at line 3-3 be cut a slot having the circular are 11 with a flat bottom 23 and with straight parallel vertical walls extending upwardly perpendicular from the arcuate fiat bottom, the tops of these walls being represented by the dotted lines 16 and 17 in Fig. 5.

From the position of symmetry in which the cutter was held while making the initial cut described above, the axis of rotation of the milling cutter 12 will now be twisted or turned in both directions, through a small angle above the axis coinciding with the longitudinal axis of the screw. This angle of twist may conveniently be about five degrees, but it will be understood that a few degrees more or less would not depart in principle or effect from the invention. Fig. 5 shows the cutter (in outline form, without showing the individual teeth) twisted through an angle of about five or more degrees counterclockwise about the longitudinal screw axis. This will correspondingly modify the slot by removing material, and will create two portions of substantially conical surfaces which are almost planar, one in the upper hemisphere of the screw head and the other in the lower hemisphere of the screw head (with reference to Fig. One of these portions of substantially conical surface is the surface 25 (Fig; '6), bounded by the perimeter 26, 27 and 28. The other portion of substantially conical surface will be similar in size and dimensions, but diagonally opposite in position; being bounded by the perimeter 29 and 30 of Fig. '5 (the perimeter line corresponding to line 27 of Fig. 6, not being shown). For convenience 'of terminology the surface 25 is herein referred to as surface A, and the. surface having boundaries 29 and 3G is referred to as, surface B. These surfaces, being conical, are surfaces of revolution.

' In a similar manner, the milling cutter 12 will then be turned clockwise and past the center position by the same angle '0 from the position of symmetry that it was turned counterclockwise; and in thus doing, there will be formed two more segmental frusto-conical surfaces of which the surface 31, herein referred to as surface C, bounded by the perimeter 32, 33 and 34, is shown in Fig. 6. (The corresponding diagonally opposite surface having the perimeter lines 35 and 36, herein referred to as surface D, does not appear in Fig. 6.) The surfaces A, B, C and D are herein referred to as engaging surfaces since they are for engagement by a driver. It is noted that the two engagin'grsurfaces on the same side of the slot are nonparallel to each other.

During this milling operation just described, there will be left uncut by the cutter the two opposite substantially triangiilarflplanes 37 and 38, after the cutter 18 has been turned in both directions off center. These substantially triangular portions will be in the form of opposite parallel planes with vertical opposed walls parallel to the longitudinal of the screw, and are the portions of the original slot which had .been cut according to Fig. 1. Due to, the small angle through which the axis of the cutter is rotated about the longitudinal axis of the screw, lines 27 and 33 will be nearly straight lines. However, since 'the leading edges 51 and 51a of the teeth generate a cone, and surface .27 is a plane which intersects the done at an angle to the cones axis which is greater than the angle between the leading edges and the cones central axis, lines 27 and 3.3 are slightly curved but nearly straight and. form a substantially isosceles triangle with the upper edge of the slot. Another congruent triangle will be formed on the opposite side of the slot.

Lines 26, 29, 32 and 35 are intersections between a plane .(i. e., the screws upper surface) and the conical surfaces formed by the cutter. Since the said plane is parallel to, the cones axis, these lines are slightly parabolic. Lines 28, 30, 34, and 36 are the lower edges of the conical engaging surfaces. They are formed by the outer pointed edge of the cutting teeth and are circular arcs. The arcs, which are disposed on opposite sides and opporite ends of the :slot, that is, lines 28 and 3t), 34 and 36,

. 4 lie in parallel planes, and when viewed in plan from directly above the screw head (Fig. '5), appear to be straight lines. These apparent straight lines are parallel to each other, and the perpendicular distance between them is equal to the perpendicular distance between the planar central regions 37 and 38.v

It will be apparent that the four equal sized surfaces A, B, C and D, which have been cut by the milling cutter 12, as represented by surfaces 25 and 31 in Fig. 3, will be undercut with reference to the original vertical walls of the slot, that is, the lines 28, 30, 34 and 36 will be undercut with reference to the upper edges 26, 29, 32 and 35 of the slot. This feature of the construction enables a driver to be used which will not tend to be forced out of the slot while applying the usual turning torque; and for this purpose, the driving end of the driver should have a configuration similar to that of the blank (Figs. 3 and 4) from which the milling cutter 12 was made.

The amount of divergence of the frusto-conical surfaces 20 and 20a is not critical; but for convenience, the angle of divergence should not be too great or else the width of the slot at the center would have to be excessively large in order to accommodate the end of the driver. A convenient angle for each of the lines 20 and 20a (Fig. 3) to make with reference to the normal to the head of the screw, can be in the neighborhood of about ten (10) degrees.

Figs. 8 and 9 illustrate the driving end of such a driver. It comprises the usual cylindrical shank or the like 40, tapered at 41 down to the driving end 42. The driving end comprises opposite parallel flat walls 43 and 44, which may conveniently be spaced apart by the width w of the milling cutter 18. At the lower ends of walls 43 and 44, the driver is converged inwardly along surfaces 45 and 450! which correspond to the converging surfaces 21 and 21a of Fig. 4. The diverging flat surfaces 46 and 46a down to the bottom of the driver correspond to the respective surfaces 20 and 20a of Fig. 3, and should be similarly dimensioned. The bottom surface 47 of the driver is a flat arcuate surface, corresponding to the fiat arcuate surface 23 of'the slot.

It will be recognized that the driver may readily be inserted into the slot in its position of symmetry, as shown by the position of the cutter in Fig. 7; and when its areuate surface 47 is brought down against surface 23 of the slot, the driver may then be turned in the slot by an angular amount corresponding to the amount of undercutting. Thus, to drive the usual righthand thread, the driver will be turned clockwise (with reference to Fig. 5); and when thus turned, there will be surface contact between the frusto-conical walls of the driver and of the slot, these walls of the slot being planes C and D. Likewise, when the driver is turned the other way to turn the screw in the opposite direction, its correspond ing opposite 'frusto-conical driving surfaces will make. surface contact with the remaining two slot surfaces A and B.

When the screw is being driven counterclockwise, the lower peripheral edges of the driver will be in engagement with the respective curved edges 34 and 36 of the slot; and since these slot edges are considerably undercut with reference to the surface of the screw head, the driver cannot be forced out by the usual component of force in the outward direction. This will facilitate the driving and obviate the necessity for the usual considerable inward pressure-which must ordinarily be exerted on a screw driver, and will furthermore avoid the usual undesired disengagement of the driver from the slot. Moreover, and perhaps more important, is the great amount of surface contact between the driver and the screw slot over the segmental frusto-conical driving areas. This will save the driver and prevent turning of the driver edges or points such as usually occurs in ordinary driving operations. Moreover, it will save the surface of the slot itself from being chewed up by the point of the driver.

It will be recognized that by my invention, I have provided a method for making a screw slot for use with slotted screw and screw driver arrangement which screw slot readily permits insertion and restriction of the driver into and out of the slot, when in the central position; but when the driver is turned off center, it will be effectively locked and held within the slot.

It should be understood that the invention is not limited to the particular configurations and arrangements shown in the drawings and described in the specification which are by way of illustration rather than of limitation. Modifications will readily suggest themselves for particular uses. For example, the arcuate slot need not terminate at the periphery of the screw head, but could, if desired, terminate within the periphery of the screw head, or even below the top of the head. Furthermore, the method is not limited to use with flathead screws but could be used with other types of screw heads, such as the well known roundhead screw. The undercut surfaces A, B, C and D need not be in the form of the particular portions of conical surfaces illustrated but instead might be given some other suitable shape. For example, planar surfaces might be used. However, if the surfaces A, B, C and D should depart materially from the constructions illustrated, it may be desirable to provide a correspondingly modified form of driver. Moreover, the arcuate base 23 of the slot need not necessarily be flat as made by the straight edges 15 of the cutter teeth, but instead, could be rounded somewhat, as if the teeth edges 15 where somewhat rounded.

It will be understood that variations in the abovedescribed method may be devised by persons skilled in the art which still will be within the scope of the invention. Therefore this method is to be limited only by the appended claims.

I claim:

1. The method of making an undercut slot in the head of a screw having a longitudinal axis, which comprises forcing into the top of said screw head a cutter having a central axis which is rotating on an axis of rotation, said central axis and said axis of rotation being coinciden said central axis and axis of rotation being substantially perpendicular to said longitudinal axis, said cutter having at its periphery first cutting means on or posite sides of the cutter which diverge from each other toward the periphery of the cutter, and transverse cutting means between said first cutting means, and then turning the axis of rotation of the cutter through an angle about said longitudinal axis, keeping the central axis of the cutter coincident with its axis of rotation, and the axis of rotation perpendicular to the longitudinal axis, whereby said transverse cutting means cuts a slot having an arcuate bottom and parallel side walls when the cutter is forced into the screw head, and said first cutting means undercut said side walls when said axis of rotation of the cutter is turned, thereby generating undercut surfaces which are surfaces of revolution.

2. The method according to claim 1 in which the angle through which the axis of rotation of the cutter is turned is about 5 degrees.

3. The method of making an undercut slot in the head of a screw having a longitudinal axis and a transverse slot with side walls in said head, which comprises introducing into the said transverse slot a cutter having a central axis which is rotating on an axis of rotation, said central axis and said axis of rotation being coincident, said central axis and axis of rotation being substantially perpendicular to said longitudinal axis, said cutter having at its periphery cutting means on opposite sides of the cutter which diverge from each other toward the periphery of the cutter, and then turning the axis of rotation of the cutter through an angle about said longitudinal axis, keeping the central axis of the cutter coincident with its axis of rotation, and the axis of rotation perpendicular to the longitudinal axis, whereby said cutting means undercut said side walls of the transverse slot when said axis of rotation of the cutter is turned, thereby generating undercut surfaces which are surfaces of revolution.

References Cited in the file of this patent UNITED STATES PATENTS 502,257 Jones July 25, 1893 2,238,960 Wilcox Apr. 22, 1941 2,286,633 McCabe June 16, 1942 2,377,114 Tomalis May 29, 1945 2,400,684 Clark May 21, 1946 2,646,829 Phipard July 28, 1953 

